Satellite navigation systems provide autonomous geo-spatial positioning with global or regional coverage. At the present, the dominating GNSS is the Global Positioning System (GPS). A GPS receiver has the ability to determine its geographic location to within a few meters using time signals transmitted along a line-of-sight by radio from satellites. However, there are situations when it is not possible or even permitted for a portable electronic device with a GPS receiver to receive the satellite signals used for positioning. For example, satellite signals might be obscured or blocked when the handheld device is operated indoors, or in difficult urban areas with sky rises blocking the satellite signal. Still further, the GPS system might have inadequate coverage in a specific geographic area, or the satellite signals may be actively jammed to prevent positioning.
In addition to satellite navigation, different network-bases solutions for employment in cellular radio systems have been suggested throughout the years, mostly based on different forms of time of arrival measurement of a signal transmitted from a base station to a terminal, or vice versa. In LTE release 9, 3GPP introduced a positioning enhancement techniques called Observed Time Difference of Arrival (OTDOA). Basically, this technology involves measurement of relative time difference of arrival of signals from different base stations, and combining the time difference data to estimate the position of the terminal by network calculations upon receiving the signals. In OTDOA a specific set of dedicated physical signals called Positioning Reference Signal (PRS) have been defined, which makes it easier for the terminal to make accurate timing measurements. One of the reasons for introducing OTDOA has been to supply a system for assisting location determination of emergency calls.